implement matchmaking lobby

Fixes #2.

game/player.go

@@ -41,6 +41,12 @@ func (p *Player) DTO() serve.Player {
 	return r
 }
 
+// Is returns whether the player has the same ID as other.
+// This exists for tests.
+func (p *Player) Is(other *Player) bool {
+	return p.id == other.id
+}
+
 type CuffState uint8
 
 const (

lobby/lobby.go

@@ -15,11 +15,14 @@ type Lobby struct {
 	mu sync.Mutex
 	// games is the set of all active games in the lobby.
 	games map[GameID]*game.Game
+	// matches is dealers waiting for a match. It MUST be unbuffered.
+	matches chan match
 }
 
 func New() *Lobby {
 	return &Lobby{
-		games: make(map[GameID]*game.Game),
+		games:   make(map[GameID]*game.Game),
+		matches: make(chan match),
 	}
 }
 
@@ -33,12 +36,11 @@ func (l *Lobby) Game(id GameID) *game.Game {
 // Start begins a new game in the lobby.
 // The caller must be able to distinguish the dealer's and challenger's conns
 // in order to provide correct game start DTOs to each.
-func (l *Lobby) Start(id GameID, dealer, challenger player.ID) GameID {
+func (l *Lobby) Start(id GameID, dealer, challenger player.ID) {
 	g := game.New(dealer, challenger)
 	l.mu.Lock()
 	defer l.mu.Unlock()
 	l.games[id] = g
-	return id
 }
 
 // Finish removes a game from the lobby.

lobby/match.go

@@ -0,0 +1,70 @@
+package lobby
+
+import (
+	"context"
+
+	"git.sunturtle.xyz/studio/shotgun/player"
+	"github.com/google/uuid"
+)
+
+type match struct {
+	dealer player.ID
+	chall  chan player.ID
+	match  chan GameID
+}
+
+// Queue waits for a match.
+// This may cause a new match to be created.
+func (l *Lobby) Queue(ctx context.Context, p player.ID) GameID {
+	select {
+	case <-ctx.Done():
+		return GameID{}
+	case m := <-l.matches:
+		// We found a dealer waiting for a match.
+		// We don't need to check the context here because the challenger
+		// channel is buffered and we have exclusive send access on it.
+		m.chall <- p
+		// We do need to check the context here in case they disappeared.
+		select {
+		case <-ctx.Done():
+			return GameID{}
+		case id := <-m.match:
+			return id
+		}
+	default: // do nothing
+	}
+	// We're a new dealer.
+	m := match{
+		dealer: p,
+		chall:  make(chan player.ID, 1),
+		match:  make(chan GameID, 1),
+	}
+	select {
+	case <-ctx.Done():
+		return GameID{}
+	case l.matches <- m:
+		// Our match is submitted. Move on.
+	case m := <-l.matches:
+		// We might have become a dealer at the same time someone else did.
+		// We created our match, but we can try to get theirs as well and
+		// never send ours, since l.matches is unbuffered.
+		m.chall <- p
+		select {
+		case <-ctx.Done():
+			return GameID{}
+		case id := <-m.match:
+			return id
+		}
+	}
+	select {
+	case <-ctx.Done():
+		return GameID{}
+	case chall := <-m.chall:
+		// Got our challenger. Create the game and send the ID back.
+		id := uuid.New()
+		l.Start(id, p, chall)
+		// Don't need to check context because the match channel is buffered.
+		m.match <- id
+		return id
+	}
+}

lobby/match_test.go

@@ -0,0 +1,49 @@
+package lobby_test
+
+import (
+	"context"
+	"testing"
+
+	"git.sunturtle.xyz/studio/shotgun/lobby"
+	"git.sunturtle.xyz/studio/shotgun/player"
+)
+
+func TestQueue(t *testing.T) {
+	const N = 1000 // must be even
+	games := make([]lobby.GameID, 0, N)
+	ch := make(chan lobby.GameID)
+	l := lobby.New()
+	for i := 0; i < 100; i++ {
+		games = games[:0]
+		for i := 0; i < N; i++ {
+			i := i
+			go func() {
+				ch <- l.Queue(context.Background(), player.ID{uint8(i), uint8(i >> 8)})
+			}()
+		}
+		for i := 0; i < N; i++ {
+			games = append(games, <-ch)
+		}
+		// Every unique game ID should appear exactly twice.
+		counts := make(map[lobby.GameID]int, N/2)
+		for _, id := range games {
+			counts[id]++
+		}
+		for id, c := range counts {
+			if c != 2 {
+				t.Errorf("game %v appears %d times", id, c)
+			}
+		}
+		// Every game should have two different players.
+		for _, id := range games {
+			g := l.Game(id)
+			if g == nil {
+				t.Errorf("game %v was created but doesn't exist", g)
+				continue
+			}
+			if g.CurrentPlayer().Is(g.Opponent()) {
+				t.Errorf("game %v matched with self", id)
+			}
+		}
+	}
+}